How do I test a single file using Jest?

Question

I am able to test multiple files using Jest, but I cannot figure out how to test a single file.

I have:

• Run npm install jest-cli --save-dev
• Updated package.json: `{ ... "scripts": { "test": "jest" } ... }
• Written a number of tests.

Running npm test works as expected (currently it runs 14 tests).

How do I test a single file, e.g. test app/foo/__tests__/bar.spec.js?

I have tried running npm test app/foo/__tests__/bar.spec.js (from the project root), but I get the following error:

npm ERR! Error: ENOENT, open '/node_modules/app/foo/tests/bar.spec.js/package.json'

Answer 1

All you have to do is chant the magick incantation:

npm test -- SomeTestFileToRun

The standalone -- is *nix magic for marking the end of options, meaning (for NPM) that everything after that is passed to the command being run, in this case jest. As an aside, you can display Jest usage notes by saying

npm test -- --help

Anyhow, chanting

npm test -- Foo

runs the tests in the named file (FooBar.js). You should note, though, that:

• Jest treats the name as case-sensitive, so if you're using a case-insensitive, but case-preserving file system (like Windows NTFS), you might encounter what appears to be oddness going on.

• Jest appears to treat the specification as a prefix.

So the above incantation will

• Run FooBar.js, Foo.js and FooZilla.js
• But not run foo.js